How do I make a GUI so players can only zoom out to a certain extent?

Players in the Players area of the game now have a new property called "CameraMaxZoomDistance". Just use a script that Edits that property when the Player joins, something like this:

Player = game.Players.LocalPlayer

function cameraZoom()
Player.CameraMaxZoomDistance = 25
end

game.Players.PlayerAdded:connect(cameraZoom)

Just make sure it's a LocalScript and you're set.

That should work, but I would use a server script in ServerScriptService or workspace. If you try to use a local script from workspace, it will not work. Local scripts only run in three areas:

In a player's Backpack (can be indirectly, through a Tool or HopperBin) In a player's character model In a player's PlayerGui

Also, in Slick's script every time a player joins the game EVERY player's CameraMaxZoomDistance is changed, which isn't as efficient as it could be. Although you could check if the CameraMaxZoomDistance is already set to 25, I would just use a server script.

game.Players.PlayerAdded:connect(function(plr)
    plr.CameraMaxZoomDistance = 25
end)